This invention relates to drag chains used for collecting sludge in sewage treatment plants and, more particularly, to such chains which are formed of thermoplastic material.
Traditionally, the sanitary chain used in sewage collection tanks has been formed of malleable cast iron. In recent years, a chain formed of thermoplastic material has been developed to replace the heavy, bulky, cast iron chain. The plastic chain has the advantages of lower weight for reduced chain pull and wear and lower sprocket wear, of greater corrosion resistance, and of lower installation cost and less power consumption of such systems.
The plastic chain which is now in use is formed as a unitary, one-piece link which has cross sections difficult to injection mold and provide sufficient strength because of design limitations inherent in one-piece molded articles. In addition, the narrow sidebar edges of such a unitary molded link do not provide other than a small area for contact with matching rims normally provided intended to help support the chain on the sprockets, a feature described in detail in U.S. Pat. No. 2,869,380. The connecting pins of the chain links of this prior art device are also formed of glass reinforced thermoplastic material and require an assembly tool for installation and removal because the pin is press-fitted in place. With press-fitting, two portions of the pin have to be larger than their cooperating openings and the opening on one side of the joined links must be larger than the other opening so the pin can be inserted without impairing the quality of the fit, thus the pin can only be installed and removed from one side of the link. This causes problems where access is restricted. An entirely plastic pin also adds significantly to longitudinal deformation of the chain over long-term use due to inherent creep of thermoplastic materials. If abrasive reinforcing fibers are added to the molding compound to increase resistance to creep deformation, the pin is not a good bearing material for a part whose main requirement is to serve as a pivot in a rotational sliding bearing arrangement.